During the last decade, chemical oxidation has been developed and demonstrated as an in-situ and ex-situ remediation approach for cleanup of contaminated sites. Remediation of soil and/or groundwater contamination using in situ and ex-situ chemical oxidation generally involves injecting oxidants and other amendments directly into the source zone, in down gradient plume or in a reactor. Oxidants commonly used with chemical oxidation include, for example, hydrogen peroxide (Fenton's reagent), potassium permanganate and ozone. More recently, sodium persulfate, an alternate oxidant, has been tested and reported in various technical papers.
The following table provides an example of main characteristic comparisons between Fenton's reagent, permanganate and sodium persulfate:
TABLE 1TechnologyFenton'sSodiumfeaturesReagentPermanganatePersulfatePhysical stateLiquidSolidLiquidMolecularOH•MnO4−SO4−•compositionRequired CatalystFerrous ironNoneFerrous ironor thermalactivationOxidation2.8 V1.7 V2.6 VPotentialReaction timeVery fastVery SlowSlow(minutes-hours)(Days-weeks)(hours-days)By-productFerric iron,MnO2Ferric ironO2 and H2Oand SO4Gas evolutionHigh (CO2)LowLowPermeability lossHighHighLowHeatHighLowLowDosage5-20%Up to 65 g/lUp to 55%concentrations@ 20° C.PetroleumGood-ExcellentPoorGood-ExcellenthydrocarbonsMetalPossiblePossiblePossiblemobilization
Permanganates have been used for many years for the treatment of wastewater. Potassium or sodium permanganates are generally used to oxidize contaminants directly without a catalyst or pH control. Although the oxidation power of the permanganate is higher in neutral conditions (e.g. pH of 7 to 8), it is still effective over a wide pH range. The oxidation reaction produces carbon dioxide, intermediate organic compounds and manganese oxide (MnO2).
The simplified stoichiometric equation that represents the permanganate chemical oxidation of organic contaminants (R) is the following:R+MnO4−→MnO2+CO2 or Rox+ . . .where Rox represents the oxidized intermediate organic compounds.
Unfortunately, MnO2 precipitation can result in a high permeability loss when used for in-situ treatment and the injection of permanganate can result in the mobilization and transformation of inorganic constituents such as chromium.
Although permanganate is generally more stable for example than Fenton's reagent, can usually migrate further into the subsurface and is thus more persistent, permanganate is generally not the chemical oxidant of choice for hydrocarbon contaminated soil remediation, as by comparison to more widely used oxidants, the use of permanganate generally results in relatively slow kinetics and low oxidation potential. For example, the oxidation potential of permanganate is approximately 40% lower than that of Fenton's Reagent. In addition, permanganate is usually not effective on a variety of petroleum hydrocarbons such as for example diesel, heavy PAHs, etc.
Accordingly, there is a need for an improved soil decontamination method using permanganate.